Re: Curiosity about rage checking

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 7 Lut, 09:17, "framefri...@gmail.com" <framefri...@gmail.com>
wrote:

> Consider the following piece of code
>
> for I in A..B loop                                         --
> <--------------------+
>     Put_Line("Assigning entry for I=" & Integer'Image(I));
> --                        |
>
> --                        |
>     X(I) := 0;    --  <== Access checking for this assignement could
> be moved  here--+
> end loop
>
> As explained in the fancy comment (I hope you are seeing it with a
> fixed-size font :-)

Fixed-size font is one thing, the fixed-length display system is
another. :-)

> the check for
> index overflow in the access to X could be done _outside_ the for
> loop, saving some computational
> effort.

Yes, but see below.

> My question is: is an Ada compiler allowed to move the check outside
> the loop?

It allowed to do so if it can prove that the check will never fail
inside the loop, in which case the programmer cannot tell the
difference.

>   1. The saving is not very important

It is!

>   2. Moving the check outside the loop changes the program behaviour

In which case it should not be done.
If the compiler can prove statically that A and B are withing the
range of  X's index type, then the behaviour does *not* change (all
checks are OK anyway). Otherwise the compiler cannot do any
optimization.

> However, the
> program is buggy anyway

Why? Exceptions are language events that not necessarily indicate
errors in the sense of not meeting the specification of the system.
Progammer might actually *expect* the exception to be risen.

In this context, optimizations can be reasonably applied only when the
compiler can prove that the visible behaviour (except performance)
will not change. [*]

There are, however, some places where it makes sense to optimize with
changing the visible behaviour - for example eliminating the
intermediate object that is used in the assignment of controlled types
makes sense even though it can lead to different behaviour.

--
Maciej Sobczak * www.msobczak.com * www.inspirel.com